Retinol (Vitamin A) and retinyl esters have long been added to cosmetic compositions to provide topical benefits. Retinol itself is unstable and is toxic with excessive use. Long-chain retinyl esters, however, are preferred because they are more stable.
The classical chemical preparation of long-chain retinyl esters involves either the reaction of retinol with a long chain acid, acid chloride or an ester or by the transesterification of a short-chain retinyl ester with a long-chain fatty acid ester. These processes use either harsh reagents or high temperatures, which can cause difficulties due to the instability of retinol or the retinyl esters to these types of reaction conditions.
There have been several reports of chemoenzymatic syntheses of long-chain retinyl esters. Many of these syntheses utilize expensive and unstable retinol as the starting material (O'Connor et. al. Aust. J. Chem. 1992, 45, 641; Maugard, et. al. J. Mol. Catal. B: Enzymatic 2000, 8, 275; Maugard et. al., Biotechnol. Prog. 2000, 16, 358; Maugard et. al. Biotechnol. Prog. 2002, 18, 424.). Retinyl esters such as retinyl acetate are much more stable and significantly less expensive than retinol, and several reports have utilized this material as the starting material for a biocatalytic preparation of long-chain retinyl esters. An unexamined Japanese Patent Application (JP 62-248495, 1987) utilized vitamin A acetate and a fatty acid in an organic solvent with a lipase modified with O-methoxypolyethylene glycol to prepare long-chain retinyl esters. Although reasonable yields of the desired products are obtained, this process requires a separate modification of the lipase for success. A process which did not require this modification would be more desirable. International Patent Application WO 2004/044212 A1 details a biocatalytic synthesis of long-chain retinyl esters from retinol or a retinyl ester and a fat or oil of animal or vegetable origins under solvent-free conditions. Unfortunately, this process utilizes elevated temperatures and results in only moderate conversions (17-44%) to the desired product, which could complicate isolation. A mild process for the preparation of long-chain retinyl esters from readily available precursors would therefore be of interest.